Liquid fuel cell system

ABSTRACT

Fuel cell system comprising at least one fuel cell which includes an anode compartment and a cathode compartment which are separated from one another by a proton-conducting membrane, further comprising a cathode feeder for delivering oxygen-containing gas to the cathode compartment, an anode feeder for delivering a liquid coolant/fuel mixture to the anode compartment, the anode compartment being disposed in an anode circuit which comprises a gas separator and a pump, and cooling of the coolant/fuel mixture circulating in the anode circuit is effected by the fuel cell which is designed for operation involving water break-through from the anode compartment into the cathode compartment. The evaporation cooling thus achieved in the fuel cell results in cooling of the coolant/fuel mixture at a steady-state operating temperature which is established in the fuel cell as a function of the membrane properties and the speed of the pump, thus obviating the need for any additional cooler in the anode circuit itself.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a fuel cell system comprising a fuel cell whichincludes an anode compartment and a cathode compartment which areseparated from one another by a proton-conducting membrane.

At present, the method most widely envisaged for converting liquidenergy sources into electrical energy in a fuel cell system comprising aproton exchange membrane (PEM fuel cell) all over the world is that ofreforming methanol in a gas generation system. This involves awater/methanol mixture being evaporated and being converted, in areformer, into hydrogen, carbon dioxide and carbon monoxide. Evaporationand reforming are very expensive in terms of the energy balance. Thisentails reduced efficiencies for the system as a whole. Moreover, gasbeneficiation steps are required to clean the reforming gas. The cleanedgas is delivered to the PEM fuel cell system. Additionally, a coolermust be provided to cool the coolant/fuel mixture circulating in theanode circuit.

A further problem is that of the water used in the reforming process.The product water produced on the cathode side does not suffice to coverthe water needed. Consequently, a separate water tank is required.

A so-called direct-methanol fuel cell system, as disclosed by U.S. Pat.No. 5,599,638, makes use of an aqueous methanol solution which reacts onthe anode side to form carbon dioxide. The fuel cell system describedthere includes a so-called stack consisting of a plurality ofinterconnected fuel cells. The anode compartment of the stack forms partof an anode circuit, comprising a heat exchanger to cool thecoolant/fuel mixture which is ducted off from the anode outlet andcontains carbon dioxide, a circulation tank in which the cooled mixtureis added to a freshly supplied coolant/fuel mixture, a gas separatorwhich is integrated within the circulation tank and has the purpose ofseparating carbon dioxide, and a pump to feed the coolant/fuel mixturefrom the circulation tank into the anode compartment via a correspondingfeeder. The oxygen- and water vapour-comprising cathode off-gas of theknown fuel cell system is passed through a water separator, theseparated water being fed to the coolant/fuel mixture which is to bedelivered to the anode circuit, and part of the remaining oxygen beingpassed to the oxidant supply for the cathode compartment.

Based on this, it is an object of the invention to provide asimpler-design, compact fuel cell system comprising a proton-conductingmembrane and having an improved overall efficiency.

In a preferred embodiment, the fuel cell system involves passing waterthrough the anode compartment into the cathode compartment, evaporationcooling is effected in the fuel cell as the water is absorbed by the hotair of the cathode compartment, said evaporation cooling being utilizedaccording to the invention to cool the anode circuit. Owing to thismeasure, the cooler which otherwise has to to be provided in the anodecircuit can be dispensed with.

In a preferred method, the fuel cell is operated in heat balanceequilibrium, i.e. the fuel cell is operated in a steady state at atemperature which, on the one hand, depends on the properties of theproton-conducting membrane and, on the other hand, can be adjusted viathe speed of the liquid pump. Depending on the duty point, thetemperature of the steady state operation is between 90 and 110° C.Setting a steady-state operating temperature is of crucial importance inincreasing the efficiency of the fuel cell or of the stack formed from aplurality of fuel cells, since this will enable isothermal operation ofthe stack, i.e. temperature differences over the length of the stack ofan order of magnitude of about 10° C., which are standard in knownsystems, will no longer occur, or only to an insignificant extent.

The inventive evaporation cooling in the fuel cell has the additionaladvantage that the mass flow of the dry air is increased by a factor of1.5 to 2, entailing an increase in expander capacity by the same factor.This also entails energy savings for air supply in full-load operation.

In a preferred embodiment, an air cooler downstream of the expander isprovided which is thermally coupled to the vehicle radiator and whichserves for condensing out water to achieve a positive water balance inthe system.

The invention is depicted schematically in the drawing with reference toa specific embodiment and is explained below in more detail withreference to the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The only FIGURE shows a schematic depiction of the basic configurationof a fuel cell system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The fuel cell system depicted in the FIGURE comprises a fuel cell 10which consists of an anode compartment 12 and a cathode compartment 14,which are separated from one another by a proton-conducting membrane 16.Via an anode feeder 18, the anode compartment 12 is supplied with aliquid coolant/fuel mixture. The fuel used in this context can be anyelectrochemically oxidizable substance having the general structuralformula H—[—CH₂O—]_(n)—Y, where 1≦n≦5 and Y=H or Y=CH₃. The fuel cellsystem of the specific example shown is operated with liquid methanol asa fuel and water as a coolant. Even though the following is restrictedto a description of the use of a water/methanol mixture, the scope ofthe present application is not meant to be limited to this specificexample. Potentially suitable coolants include, in particular, liquidsor ionic or nonionic additives to water which have good antifreezeproperties. Possible fuels include, for example, branched variations onthe abovementioned general formula, for example di- ortrimethoxymethane.

An oxygen-containing gas is passed into the cathode compartment 14 via acathode feeder 20. According to the specific example shown, ambient airis used for this purpose. In the fuel cell 10, the fuel is oxidized atthe anode and the oxygen from the air is reduced at the cathode. Forthis purpose, the proton-conducting membrane 16 is coated with suitablecatalysts on the appropriate surfaces. Protons are now able to migratefrom the anode side through the proton-conducting membrane 16 andcombine, at the cathode side, with the oxygen ions to form water. Thiselectrochemical reaction gives rise to a voltage between the twoelectrodes. By connecting many such cells in parallel or in series toform a so-called stack, it is possible to achieve voltages and currentintensities which are sufficiently high to drive a vehicle.

Formed as a product at the anode outlet is a carbon dioxide gas enrichedwith water and methanol. This liquid/gas mixture is discharged from theanode compartment 12 by an anode offtake 22. The cathode exhaust aircontaining residual oxygen and water vapour is ducted off via a cathodeoff-gas line 24. To achieve good efficiency, the ambient air is providedat positive pressure in the cathode compartment 14. For this purpose,there is disposed in the cathode feeder 20 a compressor 28 driven by anelectric motor 26 and with a supercharger intercooler 29 downstreamthereof, which compressor draws in the desired air mass flow andcompresses it to the required pressure level. In the case of operationbased on ambient air, an air filter 30 is preferably additionallyprovided in the inlet area of the cathode feeder 20 upstream of thecompressor 28. Part of the energy required to compress the ambient aircan be recovered with the aid of an expander 32 disposed in the cathodeoff-gas line 24. Preferably, the compressor 28, the expander 32 and theelectric motor 26 are disposed on a common shaft. Control of the fuelcell output is achieved by open- or closed-loop control of thecompressor speed and consequently of the available air mass flow.

On the anode side, the water/methanol mixture is circulated at apredefined pressure with the aid of a pump 34, so that an excess supplyof fuel will be ensured at the anode at all times. The ratio of water tomethanol in the anode feeder 18 is set with the aid of a sensor 36 whichmeasures the methanol concentration in the anode feeder 18. Depending onthis sensor signal, the concentration of the water/methanol mixture isthen controlled, the liquid methanol being delivered from a methanoltank 38 via a methanol delivery line 40 and being injected into theanode feeder 18 with the aid of an injection nozzle 44 not shown in anydetail. The injection pressure is generated by an injection pump 42disposed in the methanol delivery line 40. The anode compartment 12 istherefore supplied at all times with a water/methanol mixture having aconstant methanol concentration.

Then the carbon dioxide enriched with methanol vapour and water vapourmust be separated from the liquid/gas mixture ducted off via the anodeofftake 22. To this end, the liquid/gas mixture is delivered, via theanode offtake 22, to a gas separator 52 in which the carbon dioxide isseparated off. The water/methanol mixture remaining in the gas separator52 is recycled into the anode feeder 18 via a line 54.

The humid carbon dioxide gas separated off in the gas separator 52 iscooled to as low a temperature as possible in a cooler 56, furthermethanol and water being condensed out in a downstream water separator58. The remaining dry carbon dioxide with a small residual level ofmethanol is passed, via a line 60, to the cathode gas offtake 24, whereit is mixed with the oxygen-rich cathode exhaust air.

To separate as much liquid water as possible from the cathode exhaustair, a first water separator 59 is provided downstream of the outlet ofthe cathode compartment 14, and a further water separator 61 is provideddownstream of the expander 32, as much as possible of the water vapourformed on the cathode side being delivered to the expander 32. In thisarrangement, the expander 32 serves as a compact condensing turbine atwhose outlet part of the water vapour condenses out. The water collectedin the water separators 59, 61 is then recycled, via a feedback line 64with an integrated feedback pump 62, into a holding and purificationtank 50 of a subsidiary branch 48, 66 of the anode circuit. Inparticular, the holding and purification tank 50 is an ion exchanger.

Provided in the anode circuit, downstream of the anode outlet in theanode offtake 22, is a branch line 48 which runs to the holding andpurification tank 50. The outlet of the holding and purification tank 50is again connected tothe anode offtake 22, via a line 66 with anintegrated valve 68, upstream of the gas separator 52. The holding andpurification tank 50 serves to hold and to purify the water/methanolmixture from the anode compartment 12, the water separated in the waterseparator 58, and the product water produced on the cathode side andrecycled into the anode circuit via the feedback line 64. The valve 68firstly serves to prevent reverse flow from the anode offtake 22 intothe line 66, and secondly to establish that fraction of the mixture fromthe anode offtake 22 which is to be passed through the holding andpurification tank.

According to the invention, the fuel cell 10 is operated with waterpassing through the membrane 16 from the anode compartment 12 into thecathode compartment 14. The liquid water thus reaching the cathodecompartment 14 is partially absorbed as vapour, up to saturation limit,by the dry, hot air entering the cathode compartment 14 via the cathodefeeder 20. This results in evaporation cooling in the fuel cell 10, saidevaporation cooling being utilized according to the invention to coolthe coolant/fuel mixture circulating in the anode circuit. Thus thecooler which is otherwise normally provided in the anode offtake 22 canbe dispensed with.

The water passthrough is due to an electro-osmotic transport phenomenonof the membrane 16. On the anode side, water molecules cluster aroundeach proton. Electro-osmotic pressure causes the latter to migratethrough the ion channels of the membrane 16, e.g. Nafion®, to thecathode side. The number of the bound water molecules in this situationis slightly temperature-dependent and also depends on the ion channeldiameter of the membrane 16. The higher the electro-osmotic transportcoefficient of the membrane 16, the more water will reach the cathodeside, be able to evaporate there, and therefore, be able to be utilizedfor evaporation cooling of the fuel cell 10.

The transport via the membrane 16 causes about ten times more water topass into the cathode compartment 14 than is formed there by theoxidation of hydrogen. In the case of e.g. a Nafion membrane, about 5water molecules are bound to a proton which migrates through themembrane 16, whereas only one water molecule per two protons is formedin the oxidation. At 80° C., on average slightly fewer than 5, and at120° C. slightly more than 5 water molecules are bound to a proton. Inthe case of a membrane material having larger ion channels, more watermolecules can be bound to a proton, fewer in the case of a membranematerial having smaller ion channels.

The water passing through the membrane 16 evaporates on the cathode sideand cools the fuel cell 10 by evaporation cooling.

Preferably, the temperature of the cathode 14 is close to the boilingpoint of water, to evaporate as much of the permeating water aspossible, the positive pressure prevailing at the cathode 14 beingcapable of being set in a simple manner to control the boiling point ofwater. At a positive pressure of 1 bar, the boiling point is about 120°C. instead of 100° C. at atmospheric pressure. The temperature of thefuel cell is established in accordance with the positive pressureapplied at the cathode side.

The water vapour is delivered to the expander 32. It is particularlyadvantageous to prevent water vapour from condensing out en route to theexpander 32. Thus, it is preferable that the lines are thermallyinsulated in a suitable manner, to prevent the water vapour fromcondensing out. Equally, it is expedient to make allowances, regardingthe connection lines between cathode 16 and expander 32, for the largervolume required for the water vapour by making the line diameterssufficiently large.

In the fuel cell 10, owing to the operation in water-passthrough mode, asteady-state operating temperature can be set without the need of thecooler normally provided in the anode circuit. The steady-stateoperating temperature can be set by controlling the positive pressure inthe cathode compartment 14 and/or the speed of the pump 34 whichprovides the volume flow on the anode side. Advantageously, thesteady-state operating temperature is between 90 and 110° C.,particularly 105° C. This allows the fuel cell or a stack formed of aplurality of fuel cells to be operated virtually isothermally.

Evaporation cooling additionally, as already mentioned above, has theadvantage of increasing the mass flow of the dry air by a factor of from1.5 to 2. Thus the capacity of the expander 32 is increased by the samefactor, entailing energy savings for the air supply. These savings areabout 8 kW in full-load operation. An air cooler 46 disposed downstreamof the expander 32 is thermally coupled to the vehicle radiator (notshown in any detail) and has the purpose of condensing out water fromthe exhaust air stream to achieve a positive water balance in thesystem.

What is claimed is:
 1. A fuel cell system, comprising: 1) at least onefuel cell which has a) an anode compartment, b) a cathode compartment,and c) a proton-conducting membrane which separates said anodecompartment from said cathode compartment and is capable of allowingwater to pass; 2) a cathode circuit in which said cathode compartment isdisposed, said cathode circuit further including a cathode feeder fordelivering oxygen-containing gas to said cathode department; 3) anexpander unit disposed in said cathode circuit, wherein water vaporgenerated in the cathode compartment is delivered to said expander unit;4) an anode circuit in which said anode compartment is disposed, saidanode circuit further including an anode offtake connected to a gasseparator, and an anode feeder for delivering a liquid coolant/fuelmixture to said anode compartment, whereby cooling in the anode circuitis effected by evaporation of liquid coolant that passes through saidmembrane from the anode compartment into the cathode compartment, withno additional heat exchanger being provided in said anode circuit; and5) means for setting and maintaining a desired operating temperature insaid fuel cell by adjusting at least one of pressure in said cathodecompartment or a rate of delivery of the liquid coolant/fuel mixture tosaid anode compartment.
 2. The fuel cell system of claim 1, furthercomprising a compressor unit disposed in said cathode feeder.
 3. Thefuel cell system of claim 1, further comprising a compressor unitdisposed in said cathode feeder.
 4. The fuel cell system of claim 3,further comprising a supercharger intercooler, a cooler, and at leastone water separator for water recovery, wherein said superchargerintercooler is disposed downstream of the compressor unit, and saidcooler and at least one water separator are disposed downstream of theexpander unit.
 5. The fuel cell system of claim 4, further comprising afeedback line, wherein recycling of recovered water from the at leastone water separator into the anode circuit is provided via said feedbackline.
 6. The fuel cell system of claim 1, further comprising a holdingand purification tank disposed in said anode circuit.
 7. The fuel cellsystem of claim 6, further comprising a subsidiary branch of the anodeofftake, wherein said holding and purification tank is disposed in saidsubsidiary branch upstream of said gas separator.
 8. The fuel cellsystem according to claim 1, further comprising: 5) a subsidiary branchof the anode offtake, which splits off from said anode offtake, and isconnected to said gas separator; and 6) a holding and purification tankdisposed in said subsidiary branch, upstream of said gas separator.
 9. Amethod of operating a fuel cell system having at least one fuel cellwhich includes an anode compartment and a cathode compartment which areseparated from one another by a proton-conducting membrane, and an anodefeeder for delivering a liquid coolant/fuel mixture to the anodecompartment, comprising: passing coolant through the proton-conductingmembrane from the anode compartment into the cathode compartment;evaporating the coolant in the anode compartment; and adjustingtemperature in said at least one fuel cell to a desired value by varyinga rate at which said coolant evaporates in said cathode compartment;wherein said varying of said rate at which coolant evaporates in saidcathode compartment is performed by adjusting at least one of a flowrate of the liquid coolant/fuel mixture or pressure in said cathodecompartment.
 10. The method of claim 9, wherein the operatingtemperature is between 90 and 110° C.
 11. A method of controlling anoperating temperature of a fuel cell system having at least one fuelcell that includes an anode compartment and a cathode compartment whichare separated from one another by a proton-conducting membrane, and ananode feeder for delivering a liquid coolant/fuel mixture to the anodecompartment, comprising: passing coolant through the proton-conductingmembrane from the anode compartment into the cathode compartment;evaporating the coolant passing into the cathode compartment, wherebythe evaporation of the coolant cools the coolant/fuel mixture in theanode compartment; and setting and maintaining a desired operatingtemperature in said fuel cell system by varying a rate at which saidcoolant evaporates in said cathode compartment; wherein said varying ofsaid rate at which coolant evaporates in said cathode compartment isperformed by adjusting at least one of a flow rate of the liquidcoolant/fuel mixture or pressure in said cathode compartment.